I think ease of design has more to do with field of view and entrance pupil size (focal length / f-number). The easiest lens designs I have seen which are very good are large format lenses. They are very simple lenses and don't need very high resolution (since the sensor size, film, is measured in inches).

I think in general smaller sensors are harder to design lenses for, as you have to start getting excellent performance at very large apertures to make them any good.

Very true. If equivalent lenses had the same or lower production cost/spec then smaller sensors were the winners. But the reality is that by increasing the sensor size the production cost/spec for equivalent lenses goes down. I wounder how much would an equivalent 12-35mm f/2 m43 lens cost and weigh, especially if the requirements were at least 80-100 lpmm in the center and 70-90 in the edges to match the Sony lens.